The present invention relates to a power transmission device for a motor vehicle which is designed to absorb shocks due to torque variation of the engine output.
Power transmission device for motor vehicles any be defined as a device that transmits torque produced by the engine to the driving wheels and it is composed generally of a clutch, a transmission, a differential, and drive axles.
A portion of the torque variation of the engine is absorbed by inertial moments and elastic deformation of constituents of the power transmission device. However, owing to their rigidity and little flexibility, they fail to absorb torque variation of the engine when the engine operates at low speeds, allowing longitudinal acceleration/deceleration to be imposed on the vehicle. At frequencies above 40 Hz where the drive axles have little vibration damping capability even though they work most effectively in absorbing torque variation, the overall system has insufficient vibration damping capability. Thus, the free vibration caused by an abrupt change in driving force, such as at acceleration, fails to be damped quickly, inducing vibration of the vehicle body. In order to examine the vibration characteristic of the power transmission device discussed above, reference is made to a simplified model shown in FIG. 2A. The reference characters used are as follows:
Iw=the inertia moment of rotary members of an engine; PA1 Kc, Cc=the torsional spring constant and the damping coefficient of a mechanism, such as a clutch, between a flywheel and a transmission; PA1 Ks=the torsonal ring constant of a mechanism between the transmission and driving wheels, Ks=(the torional spring constant)/(the total reduction ratio, (i).sup.2. The damping coefficient provided here is substantially zero; PA1 I.sub.B =the vehicle's weight, I.sub.B =Mr.sup.2 /i (where, M: the vehicle's mass, r: the effective diameter of the tire i: the final reduction ratio).
The resonant frequency f.sub.0 of this conventional system is expressed as: ##EQU1## The dashed curve V shown in FIG. 1 shows the variation characteristics of the amplitude of torque imposed on I.sub.B when the external force (torque) T cos .omega.t is applied to Iw. The vertical coordinate represents a ratio of the transmissibility of excitation force to the transmissibility thereof computed on the assumption that the spring constant is infinity (=I.sub.B /(I.sub.w +I.sub.B)). The horizontal coordinate represents a frequency. As is readily understood from the dashed curve V shown in FIG. 1, the conventional power transmission device creates a great peak due to the resonance at around 10 Hz. Over a range of frequencies from 40 Hz to 50 Hz which corresponds to an operating condition where a 4-cycle internal combustion engine operates at speeds falling in a range from 1200 to 1500 rpm, the forced oscillation caused by torque variation produced by the engine cannot be damped to a satisfactorily low level.
Published Japanese Patent Application No. (JP-A) 54-21 (not Japanese Pat. No. 968,953) discloses a clutch for a motor vehicle. In order to eliminate substantial shocks taking place upon abrupt engagement of the clutch, flywheel is divided into two parts and a helical spring is operatively disposed between them with its one end fastened to one part and the other end fastened to the other part. A power transmission device employing this known clutch may be expressed by a simplified model shown in FIG. 2B. The two dots chain line W shown in FIG. 1 shows the variation characteristic of the amplitude of torque imposed on I.sub.B when the external force (torque) T cos .omega.t is applied to Iw. This curve W peaks at a resonant frequency falling in a range from 20 to 30 Hz, particularly at 21 Hz. Thus, the vibration of the system is brought into correspondance with the vibration of the second order when the engine operates at engine speeds falling in a range from 600 to 900 rpm, thus causing abrupt vibration to take place if the cluth is engaged upon starting the vehicle when the engine operates at low speeds. Thus, this characteristic is not satisfactory.
This known clutch tends to vibrate as a whole when a low torque is transmitted therethrough immediately after the maximum torque has been transmitted therethrough because the single spiral spring tends to have its center greatly deviated from the center of the clutch when it is unwound as a result of a rapid drop in torque after it has been tightly wound upon an application of the maximum torque. This eccentric state of the spiral spring creates a imbalance in centrifugal force and the deviation is further increased owing to the rotation of the clutch, thus causing vibration of the clutch as a whole.